PS.... I wrote:
In general, I do not believe there is any reliable way
to avoid them, since the sum is computed internally.

But the following steps, together or separately, might
remedy #NUM errors for __specific__ sets of data.

1. Reduce the number of line items (rows).

As a demonstration of the quirkiness of this approach (by itself), it might
be noted that in RK-Excel's workbook, PROB has no problem with the 346 lines
of data in M3:N346, much more than the 198 lines of data in A3:B198.